Dark mode swaps a light background and dark text for a dark background and light text, and is now a standard option across operating systems, browsers, and most modern apps. Beyond aesthetic preference, it genuinely helps people with light sensitivity, migraines, or certain visual impairments who find bright white backgrounds uncomfortable or painful to look at for extended periods.
Sites that implement dark mode need to re-check contrast ratios in that palette specifically, since colors that pass WCAG contrast requirements in light mode do not automatically pass once the background and text colors invert.